This invention relates to spectrometers of the type which, (a) incorporate interferometers in which spectral scanning is accomplished by varying the path length of radiation in one interferometer arm, and (b) use the Fourier Transform performed by a computer to convert electronic signals derived from the optical output of the interferometer into spectral analysis data.
Changing the length of one interferometer arm requires motion, either of a reflector which constitutes the end of the variable length arm, or of a refractive element interposed in the variable length arm. Controlling such motion is a difficult and expensive procedure, because it must be very uniform in order to produce reliable scanning data. Any lack of smoothness in the motion will interfere with the data output.
Generally, the solution for this problem, in high peformance spectrometers, has been the use of air bearings to support the moving element. The substantial cost of air bearings and their related structures has been unwelcome, and has prompted numerous efforts to find less expensive types of bearings. Sliding bearings have not been successful because of their tendency toward "stickiness", and ball bearings have not provided sufficiently smooth and linear motion. The use of oil is to be avoided because of the susceptibility of the instruments to contamination.
The foregoing considerations tend to limit the non-air-bearing possibilities to point, or pivotal, bearings rather than surface bearings. Such constructions have been proposed, e.g., a "parallelogram" linkage arrangement for supporting a moving reflector during its spectral scanning motion. However, for a variety of reasons, the systems proposed thus far have not been commercially successful. The problems caused by undesired tilting or shearing movement of the moving reflector have not been adequately solved. Also, the use of a parallelogram linkage creates a limitation on the width of the scan, because it tends to limit the permissible motion of the moving reflector, and thus limit the available path length variation of the analytical beam.
Furthermore, as discussed at length in Doyle Application Ser. No. 470,937, filed Mar. 1, 1983, the problem of incorporating a reference beam which signals the start of each analytical scan has not been adequately resolved. Interferometers of the type under discussion generally incorporate three radiation sub-systems: (1) the infrared (IR) radiation which is the basic analytical beam; (2) a monochromatic (laser) beam which derives pulses from a periodic fringe pattern to "clock" the sampling of detector signals by the computer system; and (3) a wide-band, or "white" light beam which is used to start each spectral scanning sweep at the identical point in the spectrum, in order that the integrated spectral data output will have maximum accuracy. Alignment of the various interferometer optics and radiation beams requires extreme accuracy. One of the major problems is any undesired change of position of an optical element which has the effect of altering the phase relationship between the white light which produces the scanning reference point and the infrared light which produces the spectral analysis data.